Page 275 - Tribology in Machine Design

260 Tribology in machine design

lubricant no longer provides sufficient protection. The lubricant film
parameter A is a measure of the adequacy of the lubricant film to separate
the bearing surfaces. In order for the frequency of asperity contacts between
the rolling surfaces to be negligible, X must be greater than 3. When / is
much less than 1, we can expect significant surface damage and a short
service life of the bearing. When A is between approximately 1.5 and 3, some
asperity contact occurs, but satisfactory bearing operation and life can be
obtained due to the protection provided by the lubricant.
Predicting the range of A for a given application is dependent on knowing
the magnitude of the elastohydrodynamic film thickness to a fair degree of
accuracy. Surface roughness can be measured but may be modified
somewhat during the running-in process. The film thickness can be
evaluated using one of several equations available in the literature. Some of
them are presented and discussed in Chapter 6.
Liquid lubricants also serve other functions in rolling-element bearings.
The heat generated in a bearing can be removed if the lubricant is circulated
through the bearing either to an external heat exchanger or simply brought
into contact with the system casing or housing. Other cooling techniques
with recirculating lubricant systems will be discussed later. Circulating
lubricant also flushes out wear debris from intermittent contact in the
bearing. Liquid lubricant can act as a rust and corrosion preventer and help
to seal out dirt, dust and moisture. This is especially true in the case of
grease.

7.5.2. Solid film lubrication
When operation of rolling-element bearings is required at extreme
temperatures, either very high or very low, or at low pressure (vacuum),
normal liquid lubricants or greases are not usually suitable. High-
temperature limits are due to thermal or oxidative instability of the
lubricant.
At low temperatures, such as in cryogenic systems, the lubricant's
viscosity is so high that pumping losses and bearing torque are unac-
ceptably high. In high-vacuum systems or space applications, rapid
evaporation limits the usefulness of liquid lubricants and greases.
For the unusual environment, rolling-element bearings can be lubricated
by solid films. The use of solid film lubrication generally limits bearing life
to considerably less than the full fatigue life potential available with proper
oil lubrication. Solid lubricants may be used as bonded films, transfer films
or loose powder applications. Transfer film lubrication is employed in
cryogenic systems such as rocket engine turbopumps. The cage of the bail-
or roller-bearing is typically fabricated from a material containing PTFE.
Lubricating films are formed in the raceway contacts by PTFE transferred
from the balls or rollers which have rubbed the cage pocket surfaces and
picked up a film of PTFE. Cooling of bearings in these applications is
readily accomplished since they are usually operating in the cryogenic
working fluid. In cryogenic systems where radiation may also be present,